Certain Faces not Appearing after .OBJ Model Export
They are not gone, they are flipped. GE don't render the backside of faces, unless you specifically tell them to. 
You can tell that's the case, because the part that's missing in Blender has a blueish color. 
Select that part, go into edit mode and hit CRTL + N
if that doesn't change anything, go back into edit mode and press CRTL + F and choose Flip Normals